In the catalytic gas phase oxidative dehydrogenation of alkyl glycolate using a silver catalyst, the higher the catalyst temperature employed, the lower the selectivity for alkyl glyoxylate. Because of the potentially low selectivity for alkyl glyoxylate at high temperatures, i.e., above about 700.degree. C., prior art processes kept the reaction temperature low, i.e., below about 400.degree. C., by, among other techniques, adding diluents. For example, U.S. Pat. No. 4,340,748 discloses a process in which glyoxylic esters are produced in high yields by the catalytic oxidative dehydrogenation of glycolic acid esters in the gaseous phase in the presence of a silver catalyst. By using 40 to 60 moles of a carrier gas per mole of the glycolic acid ester, the temperature of the reactor can be maintained at 200.degree. to 400.degree. C. Catalyst productivity measured as space time yield is, however, low due to the high degree of dilution. U.S. Pat. No. 4,340,748 discloses carrying out the reaction with an effective amount of a catalyst promoter and an inert substance (water, chlorinated hydrocarbons, etc.) to increase selectivity of the reaction. Chloroform is the only chlorinated hydrocarbon disclosed, and in Example 11 it is used in relatively large quantities, i.e., about 0.36 moles of chloroform per mole of alkyl glycolate, equivalent to 330,000 parts CHCl.sub.3 per million parts alkyl glycolate. The reaction in Example 11 has a catalyst carrier containing several metal catalysts, including silver, together with a metal catalyst promoter. The process disclosed in U.S. Pat. No. 4,340,748 may be unsuitable in certain instances where a pure silver catalyst is used, because the use of large quantities of a chlorinated hydrocarbon are known to poison or otherwise deleteriously affect the catalytic activity of silver. The process also has the disadvantage of requiring the use of an expensive catalyst promoter.
European Patent No. 149,439 recommends against using diluents when it is desired to achieve high selectivity. Instead, European Patent 149,439 minimizes the residence time in the reactor by passing the reactant stream through a catalyst bed at a very high linear velocity. In this way, the time is minimized that the reactant stream is subjected to high temperatures and catalytically active areas. Since diluents are not used, the reactor temperature increases to 400.degree.-600.degree. C. To minimize side reactions, extremely high gas velocities and correspondingly short contact times are employed so that the conversion of the alkyl glycolate is limited to less than 70 percent. This process has the disadvantage that the short contact time results in a relatively low rate of conversion of the alkyl glycolate which, in turn, makes it difficult to isolate the desired product from unreacted starting materials.
It is well known to use additives to increase the selectivity in several silver-catalyzed oxidation reactions. One theory advanced is that the additive partially poisons the catalyst, thereby minimizing side reactions. The oxidation of ethylene is one such area where the principle of catalyst poisoning to increase selectivity has been used. Many additives have been disclosed in the epoxide literature that increase the selectivity of the reaction [Kirk-Othmer Encyclopedia of Chemical Technology (3rd edition, Volume 9, pp. 441-449) which is incorporated herein by reference]. The use of catalyst inhibitors to suppress undesired side reactions is also discussed in Kirk-Othmer. Catalyst inhibitors include aromatic hydrocarbons, amines, organometallic compounds and alkyl halides.
It is well known that adsorption of oxygen on metallic silver can proceed by several different pathways depending upon many variables including temperature, partial pressure of oxygen, and the detailed structure of the silver surface. It has been theorized that different forms of adsorbed oxygen react with organic substrates by different mechanisms to produce different products. For example, it has been suggested in Kirk-Othmer, 3rd edition, Volume 9, pp. 441, that a monoatomic oxygen species adsorbed on silver reacts with ethylene to form CO2 while a co-formed diatomic oxygen species reacts with ethylene to form ethylene oxide. These reactions are shown as follows: ##STR1## In certain silver-catalyzed oxidations of ethylene, increased selectivity for ethylene oxide formation is achieved by adding to the catalyst or to the feed stream certain materials which are believed to alter the ratios of the various adsorbed oxygen species. Catalysis Review for Science and Engineering, 22(2), p. 224 (1980), discloses that other additives, such as alkali and alkaline earth metals, chlorine, sulfur, selenium, tellurium, phosphorous, and certain halogenated organics have been found to increase the selectivity.
The choice of a suitable additive to enhance the selectivity is dependent upon the detailed nature of the chemical process. In the oxidative dehydrogenation of methanol to formaldehyde in the presence of a silver catalyst, certain phosphorous compounds have been found to enhance the selectivity as in the ethylene oxide process. However, other additives which are beneficial to the ethylene oxidation, such as sulfur, alkali metals and halogenated hydrocarbons, have been found to be detrimental in the formaldehyde process. Consequently, techniques which are successful in one silver-catalyzed oxidative dehydrogenation cannot be extrapolated to another chemical system. In this connection, although the ethylene oxide technology may explain the mechanism in which oxygen is absorbed onto a silver catalyst, it cannot predict which substances will promote the oxidation of alkyl glycolate. In any particular chemical reaction, there are factors which are as important as the mode in which the oxygen is absorbed on the catalyst.
It is, therefore, desirable to have an economical process for the production of alkyl glyoxylates and provide an additive which will increase the selectivity of the alkyl glyoxylate at high rates of conversion for alkyl glycolate.